1. Field of the Invention
The present invention relates to a gain controlled amplifier with an improved S/N ratio well adaptable for integrated circuits.
2. Description of the Prior Art
As a gain controlled amplifier (GCA) requiring a wide dynamic range or a wide gain controlled range, there has been known a log/exp type gain controlled amplifier by using a logarithmic conversion and an exponential conversion. By the log/exp GCA, a wide gain controlled range is obtained indeed. When there is a poor coincidence of the exponential function characteristic between the logarithmic and exponential conversions, a poor analogy is present between the profiles of the input and output signal waveforms, however. Particularly when the characteristic of the paired transistors of NPN to control the positive half period of the input signal is not well coincident with that of the paired transistors of PNP to control the negative half period of the input signal, there occurs an asymmetircal distortion between the positive and negative periods. Generally, the frequency characteristic of the PNP transistor is poorer than that of the NPN transistor. In the GCA containing NPN transistors and PNP transistors respectively in the feedback circuits, the asymmetrical distortion becomes larger as the input signal frequency is higher. For this reason, when the log/exp type GCA is used for an audio amplifier of HiFi use, the discrete dual NPN and PNP transistors must frequently be selected in their characteristics so as to lessen the asymmetrical distortion. This makes it difficult to fabricate the log/exp type GCA by the IC.
One of the prior arts attempted solution of the problem of the log/exp type GCA, i.e. the difficulty of the IC fabrication of the low distortion type GCA, is an Automatic Gain Control RF-IF amplifier disclosed in the U.S. Pat. No. 3,684,974. In the AGC amplifier disclosed in the patent, all the active elements may be formed by NPN transistors. Further, a signal transmission path for the positive half period of the input signal is identical with or symmetrical with that for the negative half period of the input signal. Therefore, it is estimated that the AGC amplifier of the USP is easy in the IC fabrication, and has a good frequency characteristic and a little distortion. To obtain those features for advantages, however, an idling current or bias current more than a maximum peak value of the input signal current must constantly be fed into the AGC circuit. When the idling current is too small, when the signal current larger than the idling current is inputted, part of the AGC circuit is cut off to cause a clipping distortion. When the constant flow of sufficiently large idling current is allowed in the circuit for preventing the clipping distortion, an additional problem arises that the shot noise (white noise) of the active elements forming the AGC circuit increases as the idling current is larger, resulting in deterioration of the S/N ratio. The deterioration of the S/N ratio implies that the dynamic range is narrowed. Thus, the AGC amplifier disclosed in the above patent has advantages of the easy IC fabrication and the low distortion but has a disadvantage that it is difficult to take a wide dynamic range.